For me, this occurred when I provided my app's compliance information on the App Store tab. The Test Flight tab then still required compliance and the error occurred when I tried to complete it again from this tab. From the error "You cannot update when the value is already set." I am assuming the Test Flight tab is somehow not aware that the compliance is already provided.
I was able to fix the error by clearing site data (In Chrome: F12 - Application - Clear site data) and logging in again. From here, the Test Flight tab no longer asked for compliance information.
CampbellMG
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For me, this occurred when I provided my app's compliance information on the App Store tab. The Test Flight tab then still required compliance and the error occurred when I tried to complete it again from this tab. From the error "You cannot update when the value is already set." I am assuming the Test Flight tab is somehow not aware that the compliance is already provided.
I was able to fix the error by clearing site data (In Chrome: F12 - Application - Clear site data) and logging in again. From here, the Test Flight tab no longer asked for compliance information.